1. Field of the Invention
The present invention generally relates to methods for removing a photoresist mask from an etched substrate. More specifically, the present invention relates to an improved method for stripping a photoresist mask used in CH.sub.4 /H.sub.2 reactive ion etching (RIE) of semiconductors, particularly those based on InP and GaAs.
2. Description of Related Art
The efficient and effective removal of photoresist masks used during the reactive ion etching process of semiconductors continues to be of concern. Harsher stripping methods have been needed as photoresist masks have become harder to remove. These harder photoresists have come about both intentionally and unintentionally. An intentionally hardened photoresist may be desirable, as indicated by Allen et al., "Deep U.V. Hardening of Positive Photoresist Patterns." J. Electrochemical Soc., Vol. 129, No. 6, pp. 1379-1381 (1982). Therein, deep U.V. curing by flood exposure was employed for the purpose of making the photoresist less susceptible to thermal deformation.
More frequently, a harder photoresist is created unintentionally. That is often due to the photoresist being subjected to harsh semiconductor production processes. For example, high dose ion implantation processes create a harder resist. As another example, the resist often undergoes high temperature baking to obtain better anisotropic etching, but this also creates a harder resist. In yet another example, and in the context of RIE with carbon containing compounds, the resist tends to become harder to remove due to polymerization of the resist surface.
To remove the resist, dry and wet methods have been developed with varying advantages and disadvantages. Dry methods typically volatilize and remove the bulk of the resist. One such dry method uses a barrel asher which places the wafer directly in a low frequency or rf plasma. Another dry method uses a downstream asher which forms a plasma away from where the wafer is positioned. However, when a downstream or barrel asher is used alone, a residual amount of resist may remain which necessitates subsequent processing for complete removal. Alternatively, harsher conditions such as higher power and/or temperature may be needed to remove the residual resist.
Past wet methods have employed inorganic and organic solutions. A typical inorganic solution has been 2:1 H.sub.2 SO.sub.4 /H.sub.2 O.sub.2 for the silicon industry. However, due to incompatible chemistries, that solution cannot be readily used in III-V compound semiconductors. Organic strippers have often been used in conjunction with megasonics or ultrasonics. However, the accompanying agitation aspect can be detrimental to devices with submicron features and to devices fabricated on III-V semiconductors as they are relatively brittle.
Apart from the traditional dry and wet methods directed specifically for stripping resist, there have been methods of removing sidewall polymers formed during the etching process. As noted above, and in a related fashion, etching processes may tend to polymerize the surface of the photoresist and thereby harden the resist. U.S. Pat. No. 5,567,271 generally describes an oxygen RIE plasma method for removing oxidized photoresist residue. But that disclosed method is concerned with adding hydrogen as a reducing material in a non-explosive fashion. In the context of removing sidewall polymers on deep etches (i.e., &gt;5 .mu.m), a series of cycles having short etch plasmas followed by oxygen clean plasmas has been used. [J. E. Schramm, "Reactive Ion Etching of Indium-Based Compounds Using Methane/Hydrogen/Argon," Dissertation, University of California, Santa Barbara, p. 67 (1995)]. However, rather than being concerned with mask removal, such method is directed towards minimizing profile degradation.
Regardless of the particular method employed, as the stripping methods have become harsher or more aggressive, the concern over the potential resulting damage to the devices has grown. This concern has been particularly so in the context of devices based on compound semiconductors and the more recent deep submicron Si-based complementary metal oxide semiconductor (CMOS) devices which tend to be surface sensitive. What is meant by a "surface sensitive" device is one which has a greater potential of deterioration in device characteristics due to adverse effects to the semiconductor surface, which may be a result of harsh stripping methods. Particular types of devices fabricated in InP or GaAs technology that would be considered surface sensitive include high electron mobility transistors (HEMTs), heterojunction bipolar transistors (HBTs) and resonant tunneling diodes (RTDS) as examples.
The increasing interest in compound semiconductors, particularly those based on InP and GaAs, is in part due to the possibility of fabricating devices with dramatically improved speed, noise and power characteristics when compared with devices in the Si system. With those substrates, CH.sub.4 /H.sub.2 /Ar has been widely used in standard RIE, as well as in electron cyclotron resonance (ECR) and magnetically enhanced RIE (MRIE). In general, the wide use of CH.sub.4 /H.sub.2 /Ar in an InP system, for example, stems from the capability of etching without etch degradation due to volatility problems associated with other chemistries. SiO.sub.X (such as SiO.sub.2 ) and SiN.sub.x (such as Si.sub.3 N.sub.4 )have been commonly used as a dielectric mask in conjunction with CH.sub.4 /H.sub.2 /Ar as an etchant for InP and GaAs substrates. A photoresist mask, however, has not been widely used because of various reasons, including a potentially hardened top surface of the resist which results from polymerization in the carbon based CH.sub.4 /H.sub.2 /Ar RIE system. Nevertheless, when comparing the number of processing steps required for past methods of SiO.sub.x or SiN.sub.x as etch masks versus photoresist, it is apparent that the former requires more steps. Generally, more steps are required to deposit, etch, and then strip the nitride. Additional processing steps can, therefore, translate into more time and expense for a SiO.sub.x or SiN.sub.x mask when compared to a photoresist mask.
As can be seen, there is a need for an improved method of stripping photoresist used as an etch mask on substrates, such as InP and GaAs. Also needed is an improved method of stripping a mask from a substrate while reducing the number of processing steps. There is a further need for a method which can strip a photoresist mask completely (when seen under microscopic examination) from a substrate while not having to resort to aggressive dry or wet stripping methods which might otherwise cause damage to the substrate.